The desulphurising of iron melts outside of a blast furnace by injection metallic process is a well established process in the production of steel. For the reduction of the sulphur content in crude iron, in practice two processes have proved to be especially useful, namely, desulphurisation in a torpedo ladle and treatment of the iron melts in a charging ladle in the steelworks. According to both processes, desulphurisation is carried out by injection metallurgically, i.e. the desulphurisation mixture is blown into the crude iron melt through an immersion lance using a current of an inert gas.
In practice, for carrying out this process, desulphurising mixtures of calcium carbide and magnesium, possibly with further additive, for example gas developing carbon, alkaline earth metal carbonates, calcium oxide, calcium fluoride or calcium cyanamide, have proved to be advantageous. By way of example, reference is made to the following publications: German Offenlegungsschriften Nos. 25 31 047; 26 50 113; 27 08 424; 27 41 588; 35 44 562 and Stahl und Eisen, 105, No. 11, 627-630/1985.
A disadvantage of some of these processes is the use of a mixture of materials which is not mixturestable. Thus, magnesium, which makes possible a rapid and precise desulphurisation of crude iron, cannot be dosed in its amount with the necessary exactitude. Insofar as the above-mentioned processes provide for a separate addition of the magnesium to the desulphurising mixture in the form of co-injection, a relatively great expenditure for apparatus is needed in order to be able to dose the components exactly. In order in the case of co-injection to be able to dose precisely, magnesium is frequently used in admixture with slag, with aluminum shavings or other oxidic compounds. These agents also separate and, therefore, do not solve the problem.
For the above-mentioned reasons, attempts have not been lacking to produce magnesium-containing desulphurisation agents in the form of pellets, filled wires or by coating which are to impart to these agents a simplified handling and an increased process effectiveness. Such a desulphurisation agent is described, for example, in German Auslegeschrift No. 12 99 670. It has a multi-layer construction and, besides magnesium and calcium carbide, can contain further desulphurising components. However, the dimensions and compositions of the pressed bodies described in this publication are unsuitable for use in a co-injection process.
Furthermore, from German Offenlegungsschrift No. 24 22 072 is known a desulphurisation agent based on calcium carbide which is coated with magnesium metal. According to the process of this publication, the coating of the granulated calcium carbide takes place by deposition of magnesium vapour. From the actual economic point of view, the process is too laborious and too cost-intensive in order to be able to use this product for co-injection, even in the case of possible technical suitability.
The desulphurisation by means of a calcium carbidemagnesium mixture in the form of a filled wire or in the form of rods, as can be gathered from German Offenlegungsschrift No. 27 38 379, cannot be used in the co-injection process for obvious reasons.
Finally, U.S. Pat. No. 4,541,867 discloses the production of a granular agent coated with carbon which can be used as an additive for steel baths and for the desulphurisation thereof. The agent can, inter alia, consist of magnesium and calcium carbide. The coating is produced by mixing the components of the agent with a polymerisable oil, thermal polymerisation thereof and subsequent partial thermal decomposition thereof. This production process also requires a relatively high expenditure for apparatus and a high use of energy.
In general, it is also to be pointed out that because of the high magnesium vapor pressure at the temperatures of the iron melt, the introduction of the magnesium into the melts gives rise to problems. Therefore, a good dosability of the magnesium component is important for the process but has not been satisfactorily fulfilled by the desulphurisation agents at present known. Especially when magnesium is blown in without the co-injection component calcium carbide, it results in the ejection of iron and in blockage of the lances used. A disadvantage of the general nature of known mixtures is the fact that they have a content of filling materials of from 20 to 34% by weight which do not participate in the desulphurisation. A further disadvantage of known mixtures of magnesium with filling materials, such as aluminum, aluminum oxide or ball mill dust, is the fact that these separate relatively quickly so that the proportion of magnesium is initially high but thereafter decreases. Because of this inhomogeneity, a greater use of material is necessary but with a poorer action.